1. Field of the Invention
The present invention is directed to a combustion chamber design for propellant charges used in powder actuated devices, and, in particular, to a novel orifice plate which increases combustion efficiency and cleanliness. The combustion chamber promotes complete combustion of the propellant charge thus enabling the use of a power adjustment system which is both clean and has high energy consistency.
2. Description of the Prior Art
The predominant design for propellant (also known as powder actuated) charges which are currently available features a cylindrical brass casing which contains the propellant material and an ignition material. The propellant is a granular, flake, or fibrous form of nitrocellulose with additives. The open end of the brass casing is usually crimped while the opening is sealed using a wax-like substance. Ignition is attained by a technique known as rim fire. On the closed end of the brass casing, a rim area is formed. A percussion sensitive material is coated on the internal surfaces of the rim. When the firing pin impacts and collapses the rim, the percussion sensitive material reacts, and the gaseous decomposition products proceed to ignite the propellant starting with the grains or flakes which are adjacent the rim. The resultant gases and heat produced increase the pressure in the cartridge causing the sealed end of the cartridge to open. The outrush of gases from the casing carries unspent propellant into the volume on top of the driver. When the unspent propellant is forced into the clearance between the driver and its bore, the combustion is suppressed due to the reduction of pressure and temperature. The high ratio of surface area to volume in the driver-bore clearance area robs the gas/solids mixture of heat, which also contributes to the cessation of combustion. Thus, as the driver travels down the bore, unspent propellant combustion products are deposited. A number of undesirable results stem from this process: the bore can be fouled; power variance is increased due to the randomness of the incomplete combustion; and the unspent propellant can be scraped up on the return of the driver thus increasing the propellant mass of the subsequent shot. All of this results in higher shot to shot energy variance. Examples of this prior art are shown in U.S. Pat. No. 4,493,376.
Another type of propellant charge is termed caseless in that a brass casing is not used as a container for the charge. Combustion cleanliness is limited in this type of tool due to a mechanism similar to that of cartridge tools; that is, during combustion, the propellant is pushed down the bore into the clearance between the driver and the bore where combustion is suppressed due to the reduction of pressure and temperature. Examples of this art are shown in U.S. Pat. Nos. 5,208,420, 3,899,113, and 3,973,708.
Power adjustment is provided in some prior art powder actuated tools by means of a mechanical linkage which adjusts the initial position of the driver in its bore. Full power is attained when the driver is allowed to assume its initial position closest to the cartridge. Note that a minimum initial volume is provided in order to limit the peak combustion pressure. Increasingly lower power settings are achieved as the driver is initially positioned further down the bore and thus away from the cartridge. As a result of positioning the driver down the bore, two power reducing mechanisms are executed. The first mechanism is that the initial volume is increased; thus, lower combustion pressure and temperature are achieved, but also less combustion efficiency. The second mechanism is that as the driver is positioned closer to the tool's exhaust port, the chambered length is reduced. Both mechanisms have limitations and cause related problems. Increasing the initial combustion chamber volume aggravates the combustion of the propellant in that the pressure and temperature are reduced and also the surface area is increased; thus, more propellant remains uncombusted. There is a limit as to how much the power can be reduced by varying the initial combustion volume since, if the volume is too great, the propellant merely falls out of the casing, and very little combustion takes place except in the area of ignition. Repeating this procedure allows accumulation of unspent propellant which fills the intial volume and results in a dangerously large propellant mass. Meanwhile, reducing the chambered length reduces the time and distance available for combustion to take place. Also, the effect of collecting unspent propellant on the return of the driver is increased, thus causing more shot to shot energy variance. Another problem is that the exhaust gas contains a higher concentration of hydrocarbons which can be toxic. On tools of this type, unspent propellant is observed on internal surfaces of the tool's exhaust area, which can accumulate and result in an unsafe condition. Examples of this type of prior art are shown in U.S. Pat. Nos. 4,877,171; 3,746,235; 4,068,790; 4,374,567 and 4,824,003.
Another type of power adjustment mechanism operates by a valve which vents the combustion gases to either the atmosphere or in an exhaust chamber or muffler. The shortcomings of this venting technique are similar to the volume adjustment technique in that inadequate time, pressure and temperature are achieved in order to fully combust the propellant; thus, the resultant unspent propellant and by-products may foul the tool. This type of system is taught in U.S. Pat. No. 4,119,257.
Consequently, a need exists for a combustion chamber design that can completely combust the propellant even when a power adjustment means is employed.
It is thus an object of the present invention to provide a combustion chamber which combusts completely and cleanly with a sustainably low level of solid combustion products which can be carried out of the combustion chamber with the flow of the gaseous combustion products.
It is also an object of the present invention to provide a power adjust means which in conjunction with said combustion chamber promotes clean and complete combustion at all power levels.
It is further an object of the present invention to provide a combustion chamber which in conjunction with traditional power adjust means promotes complete and clean combustion at all power levels.